The present invention relates to a fixing apparatus for an electrographic recorder and more particularly to a fixing apparatus which fixes a toner to a material to be printed by using a heat roller.
Conventionally, a heat roller fixing apparatus is widely used as a fixing apparatus for an electrographic recorder which employs an electrographic process such as one used in an electrographic copy machine. A laser printer is a typical electrographic recorder.
FIG. 1 is a cross sectional view of a laser printer having a conventional heat roller fixing apparatus. A paper cassette is detachably mounted to a main body 12 of the printer. A sheet of paper 2 fed by a paper supplying apparatus 1 is conveyed with appropriate timing to a latent image carrier, comprising a photoconductor 4 on a drum, by a pair of register rollers 3. The photoconductor 4 is rotated counterclockwise, and during this rotation, the surface of the photoconductor is charged by an electronic charger 5. A laser beam from a laser system 6 is applied to the surface of the photoconductor 4 to an electrostatic latent image to be formed on the photoconductor 4. This latent image is made visual by toner when it passes through a developing apparatus. This visual image is transferred to the paper 2, which has been fed to the photoconductor 4, by a transfer-separation charger 8. The paper 2, which has been adhered to the photoconductor 4, is electrostatically separated from the photoconductor 4 and then conveyed to a fixing apparatus 9. The visual image transferred to the paper 2 is fixed thereon and is then fed to a discharge part. In the meantime, the photoconductor 4, in a state where the transferring of the image is complete, is cleaned by a cleaning apparatus 11 which has a cleaning blade. Toner removed from the photoconductor 4 is collected in the cleaning apparatus 11.
In recent years, materials printed by an electrographic recorder, particularly for a laser printer, have become diversified. Specifically, the demand for printing an image on a layered material such as an envelope has been increasing. Such a material as an envelope does not have a uniform thickness because it comprises a plurality of papers stacked on top of each other, each paper being adhered at its edges to another paper. When such a layered material is supplied to a heat roller fixing apparatus, which apparatus comprises a rigid fixing roller equipped with a heat source and a pressing roller jacketed with an elastic outer layer, wrinkles is usually formed on the layered material. Further, the quality of the picture printed on such a material does not reach a usable level, e.g. bad quality fixing, because the material is too thick.
Measures have been taken to eliminate these problems, such as reducing the hardness of the pressing roller. However, particularly in a high speed recorder, an optimum condition in which no wrinkles are formed, and with a sufficient fixing, has not been obtainable. With the above in mind, a method has been employed in which a plurality of pressing rollers are used to allow a material for printing to contact a fixing roller within a "nip" portion of each roller. The "nip" portion is a portion of the pressing roller deformed along the outer surface of the fixing roller by pressing the pressing roller to the fixing roller. By having a plurality of pressing rollers, a material for printing can be brought into contact with the fixing roller at the "nip" portion of each pressing roller, thus allowing a pressing force of each roller to be decreased. Therefore, forming of wrinkles can be prevented.
The above mentioned method is disclosed in Japanese Laid-open Patent Applications No. 55-29822 and No. 54-143145 and Japanese Laid-open Utility Model Applications No. 58-26058 and 59-66256. However, in a case where a printing material not easily torn, such as an envelope or a thick paper is supplied to a fixing apparatus having a plurality of pressing rollers, the printing material may receive a shock when it collides with a second pressing roller after the printing material is ejected from the "nip" portion formed between a first pressing roller and a fixing roller. Whether such a collision occurs depends upon the angle of motion of the printing material onto the roller. The influence of this collision appears as a "jitter" (a shear in printing) and causes a deterioration of the printing quality.
A method is suggested in Japanese Laid-open Patent Application No. 63-274968 in which speeds of two pressing rollers are respectively controlled. As a result, an appropriate tension force is applied to the material so as to prevent the occurrence of wrinkles. However, this method has a problem in that the fixing apparatus becomes complex due to each pressing roller being independently controlled by a respective power source.
Japanese Laid-open Patent Application No. 50-2447 discloses a construction, as shown in FIG. 2, wherein two pressing rollers, each having a plurality of short rollers, are arranged in such way that the short rollers of each of the two pressing rollers are alternately placed. Pressing rollers are arranged in this way so that a printing material can be easily separated from a fixing roller. In addition it is considered that occurrence of wrinkles can be prevented by allowing a slippage of the paper escaped on the both sides of the pressing roller.
However, there is a problem in that insufficient fixing may occur if a gap is formed between the alternately placed short rollers of each of the pressing rollers. Similarly, an image irregularity may occur in a portion where alternately placed short rollers overlap.